1. Field of the Invention
The present invention relates to a technique employed in an image forming apparatus such as a laser printer, and more particularly to a technique of controlling the temperature of a fuser roller fusing an image to a receiver medium such as a print sheet.
2. Description of the Related Art
Typically, an image forming apparatus such as a laser printer is operated in combination with a fuser unit. The fuser unit is provided for heating a print sheet bearing an unfused toner thereon and then fusing the toner image to the print sheet.
The above fusing unit is typically configured to include: a fuser roller incorporating a heater therein; a pressure roller disposed in pressure contact with the fuser roller; a temperature sensor (e.g., a thermistor) detecting the surface temperature of the fuser roller; etc. In printing operation, the fuser unit controls the heater in an on-off manner based on a signal from the temperature sensor, to thereby perform a fusing operation such that the surface temperature of the fuser roller is maintained at a predetermined level.
In operation, such an image forming apparatus performs a warm-up operation once a printing operation becomes required soon after the occurrence of a cold start of the fuser unit. The term “cold start” may be defined to mean a start immediately from a power-on event of the image forming apparatus, a start immediately from the release of an operation mode of the image forming apparatus from a sleep mode, etc.
Conventionally, even when the temperature of the surface of the fuser roller reaches a desired fusing-temperature owing to the above warm-up operation, the remaining portion of the image forming apparatus (possibly including the remaining portion of the fuser roller and the remaining portion of the fuser unit) fails to become adequately warm. For the reason, once the printing operation begins, the heating process of the fuser roller runs short due to heat scattering and lost into the above remaining portion and the print sheet, possibly resulting in an inadequate fixing of the print sheet.
For avoiding the inadequate fixing, it is conventional to take an approach to inhibit a printing operation during a given length of period elapsed from the occurrence of the power-on event of the image forming apparatus, or an approach to temporally maintain the temperature of the fuser roller at a temperature higher than the desired fusing temperature, i.e., a desired operating temperature. The latter approach is disclosed in Japanese Patent No. 3,119,690.